In an image sensing apparatus such as a digital camera, only an aperture mechanism for changing the aperture diameter is conventionally set. If the aperture is stopped down too much in order to widen the photographing brightness range up to a bright object, so-called diffraction occurs, decreasing the resolution. To solve this, there is recently provided an image sensing apparatus having an ND (Neutral Density) filter as a removable light-reducing optical filter. The ND filter is an optical filter with nonselective transmittance that almost uniformly transmits (reduces) wavelengths in the visible spectra of a beam.
In the signal processing circuit of the image sensing apparatus that generates image data, signal processing based on the brightness value for forming an image on an image sensing element is not changed in accordance with insertion/removal of the ND filter, posing the following problems.
In general, a brightness value Bv is calculated as a value obtained by subtracting the speed value (Sv) of an image sensing element from the sum of the aperture value (Av) and time value (Tv):Bv=Av+Tv−Sv  (1)
For example, assuming that correct exposure can be obtained at an F-number of F5.6 (Av=5), a shutter speed of 1/1000 (Tv=10), and film speed of ISO 100 (Sv=5), the brightness value Bv=5+10−5=10 is calculated from equation (1). The image sensing apparatus transfers the brightness value calculated using equation (1) to a circuit block which performs various signal processes on the basis of the brightness value. The circuit block includes a WB (White Balance) processing block which follows a change in color temperature, and an exposure value setting calculation block which calculates an aperture value, shutter speed, and the like to set an exposure value. With these blocks, the image sensing apparatus realizes an automatic white balance function, automatic exposure function, and the like.
When the ND filter is inserted, a conventional image sensing apparatus does not consider the influence of the inserted ND filter in calculating the brightness value (or exposure value). For example, assuming that correct exposure is obtained at the neutral density ND of the inserted ND filter=3, the F-number and shutter speed are F5.6 (Av=5) and 1/128(Tv=7), respectively. The image sensing apparatus calculates the brightness value Bv=5+7−5=7 in accordance with equation (1). That is, the image sensing apparatus determines that the brightness value has merely decreased (without using any ND filter), and thus transfers the brightness value to the WB processing block or exposure value setting calculation block.
A concrete example of WB processing will be explained with reference to FIG. 4. FIG. 4 is a graph showing conventional WB processing. In FIG. 4, the ordinate represents a fluorescent lamp degree Cy which is a value representing the degree of possibility that the light source is a fluorescent lamp. The abscissa represents a color temperature Cx. The curve in FIG. 4 is a WB white axis which is a target line to be followed upon a change in color temperature. The dotted line in FIG. 4 represents a WB setting range which sets a range where the WB white axis is followed. The WB processing block generally uses the brightness value Bv in photographing as one control parameter. For example, photographing at a large brightness value Bv is determined as outdoor photographing, and WB processing is so restricted as to cope with a low color temperature. To the contrary, photographing at a small brightness value Bv is determined as indoor photographing is determined as indoor photographing in which an object is illuminated with tungsten light or a fluorescent lamp, and WB processing is so restricted as to cope with a high color temperature. In this way, the influence of a color object is suppressed. In WB processing based on the brightness, if the calculation value of the brightness value Bv changes for the same object, WB processing also changes, failing to obtain image data by WB processing optimal for the object.
A concrete example of exposure value setting processing will be explained with reference to FIG. 5. FIG. 5 is a graph showing conventional exposure value setting processing. In FIG. 5, the ordinate represents the brightness of an adjusted image which is the exposure value of the image sensing apparatus. The abscissa represents the brightness value Bv. In exposure value setting processing, as shown in FIG. 5, high-brightness luminance increasing processing of increasing the exposure value for a large brightness value Bv (e.g., Bv value=8 to 9 or more) is performed. More specifically, when the brightness value Bv is large, an atmospheric scene is often obtained by relatively brightly reproducing an image at a summer seaside, winter skiing ground, or the like. For this reason, the exposure value is set slightly large for a large brightness value Bv, as indicated by the arrow in FIG. 5. If the calculation value of the brightness value Bv changes for the same object, the high-brightness luminance increase amount also changes. Image data by exposure value setting processing optimal for the object cannot be attained.
As described above, if the calculation value of the brightness value Bv changes upon insertion/removal of the optical filter (ND filter) even for the same object, processes using the brightness value Bv, such as generation of an optical system control signal and control of signal processing in a signal processing circuit, are no longer optimal for the object.